Molten metal pumps are widely used in foundries and smelting facilities to convey molten metal from a melting or holding furnace to subsequent casting or metal forming stations. One example of such a molten metal pump apparatus is disclosed in applicant's Canadian Patent No. 1,264,126, issued Jan. 2, 1990, entitled "Dual Volute Molten Metal Pump and Selective Outlet Discriminating Means," which is incorporated herein in its entirety by reference.
Normally, such a molten metal pump apparatus includes a motor mounted above a molten metal bath. The motor drives a rotatable impeller pump having one or more impellers submerged in the bath. In operation, the rotating impellers draw molten metal from the bath and pump it through a conduit routed to a subsequent station for further processing. The impellers are coupled to one end of a vertically oriented impeller shaft. An opposite end of the impeller shaft extending above the molten metal bath is affixed to a female coupling member. In pump apparatuses of conventional design, the end portion of the impeller shaft is threaded. To secure the impeller shaft the coupling member, the impeller shaft is screwed into a correspondingly internally threaded portion of the female coupling member. An end of a drive shaft extending from the motor is received in the coupling member and pinned thereto providing a mechanical linkage between the rotating motor drive shaft and the pump impellers.
Suspended in the molten metal bath are a variety of solid objects including unmelted raw materials, occluded impurities and insoluble foreign materials such as firebrick spalled from the furnace walls, chunks of cement, metal oxide accretions, etc. If a sufficiently large object is drawn into the impeller pump, the object may become lodged between the rotating impellers and a pump housing surrounding the impellers thereby blocking the continued rotation of the impellers. Blocking the impellers results in a twisting or torsional force being applied to the rotating components of the pump apparatus, i.e., the motor drive shaft, coupling member, impeller shaft and impellers. If the torsional force applied to a component exceeds the maximum or fracture point torsional strength of the component, it will fail. The fractional point torsional strength of a component is the magnitude of torsional force that causes failure of the component.
Even if the lodged object only momentarily jams the impellers and then passes through the pump without causing catastrophic failure of a pump apparatus component, the momentary blockage results in a torsional force being applied to the rotating components. Repeatedly subjecting a pump apparatus component to a torsional force near its fracture point torsional strength value will weaken the component thereby increasing the probability of premature failure.
The repair or replacement of a failed pump component is a costly, time consuming, and potentially dangerous task given the proximity of the hot molten metal. For example, if the impeller shaft fractures, the portion of the impeller shaft below the fracture point must be removed, along with the impeller pump, from the molten bath and the shaft portion disengaged from the impeller pump. Additionally, the portion of the impeller shaft above the fracture point must be unscrewed from the coupling member.
Although some steps can be taken to reduce the number and size of solid objects drawn into the impeller pump, for example, by limiting the size of the entry ports to the pump impellers and/or providing a screen mesh over the impeller entry ports, it is virtually impossible to prevent all objects capable of jamming the impellers from entering the pump without unduly restricting the entry of molten metal to the impellers.
Based on the foregoing, an improved molten metal pump apparatus that protects against both catastrophic failure and torsional force induced weakening of expensive pump components remains an objective of designers of such pumps.